1. Field of the Invention
The present invention relates to a dummy plate precursor for planographic printing used in multi-color printers, and in particular to a dummy plate precursor for use in computer-to-plate devices for producing printed plates directly. The invention also relates to a method for producing a planographic printing plate and a dummy plate for planographic printing in a common CTP plate-making system (CTP developed plate-making system).
2. Description of the Related Art
In the polychromic printing field, a dummy plate, which has no image portion, is attached to the non-image portion of the printing drum of a printer. A product obtained by completely removing a photosensitive layer from a PS plate has long been used as the dummy plate. However, in obtaining such a dummy plate, it is difficult to completely remove the photosensitive layer from the PS plate, and undesirably remaining portions of the photosensitive layer cause printing stains. In order to overcome this problem, use of a dummy plate having, on a support having no photosensitive layer thereon, a protective film for preventing ink adhesion was proposed (e.g., Japanese Patent Application Laid-Open (JP-A) No. 3-175090]. However, the dummy plate having a protective film requires an increased number of printed sheets of paper before complete removal of undesired ink adhered on the dummy plate (generally called waste of paper caused by black faults) during the initial period of printing. Thus, improvement with respect to this point has been desired.
Reasons for such waste are that the amount of protective film is much higher than that of the gum protection of common PS plates so as to prevent the printing stains even after long-term storage and that the solubility of the protective layer in the dampening water during printing decreases due to the long-term storage. Therefore, supports having no protective film have been supplied to the market by PS plate makers and is used after being hydrophilized and gummed immediately before printing in a similar manner to the PS plates. A shortcoming of such an uncoated support for a dummy plate is that it often causes printing stains due to various contaminations during long-term storage. The support is also vulnerable to damage during handling before they are completely gummed.
Dummy plate precursors having a non-photosensitive layer including a binder on a support have been used to overcome the aforementioned problem. For example, a dummy plate precursor having a non-photosensitive water-soluble layer containing a polymer having a water solubility of 2 g/L or more and an inorganic acid on a support that has been subjected to anodic oxidation or a hydrophilizing treatment (e.g., Japanese Patent No. 2,951,907), and a dummy plate precursor having a metal support having a hydrophilic surface, an undercoat layer of a water-soluble compound, and a water-insoluble and alkaline developing solution-soluble non-photosensitive resin layer (e.g., JP-A No. 11-240266) have been disclosed. Such dummy plate precursors have an advantage in that even when supplied to a CTP plate-making system common to ordinary planographic printing plate precursors, they can be developed in an automatic developing machine and converted to dummy plates, and thus are rational and economical.
On the other hand, digital technology of processing, accumulating and outputting image information electronically by computer has become widespread in recent years, and computer-to-plate (CTP) technology of producing printed plates directly, without use of a lith film, by scanning a printing plate precursor with high-directivity light such as a laser beam according to the digitalized image information has been established. For example, planographic printing plate precursors having, on a hydrophilic support, an oleophilic photosensitive resin layer containing a photosensitive compound that can generate an active species such as radical or Bronsted acid by laser exposure have been marketed as a planographic printing plate precursor allowing scanning exposure.
Accordingly, there exists a need for a dummy plate precursor that can be fed, conveyed and processed in such a CTP device without problems. In addition, an image plate and an entirely non-image plate are sometimes used in combination in the newspaper market, and a photosensitive planographic printing plate is usually used as the image plate, while a dummy plate is used as the entirely non-image plate. Namely, a system for multicolor printing of newspaper has multiple drums, and if printing does not require use of all the drums, a dummy plate is attached to at least one drum that is unnecessary for the printing.
A certain number of planographic printing plate precursors are supplied to the CTP plate-making system as a pile in which they are piled up on a base substrate called a stack board. Also, a certain number of dummy plate precursors are supplied to the CTP plate-making system as a pile in which they are piled up on another base substrate. When the making of printed plates is initiated in the CTP plate-making system, each component, i.e., a planographic printing plate precursor, a dummy plate precursor, interleaving paper, or a base substrate, should be identified by a sensor, and processing which corresponds to the identified plate is selected on the basis of the identification results. However, the above components are not securely identifiable, leaving the planographic printing plate precursor, the dummy plate precursor, and the base substrate undifferentiated. Thus, deteriorated plate-making efficiency is obtained and complicated operation is required.
Accordingly, there exists a need for a dummy plate precursor that does not cause printing stains after long-term storage, allows automatic differentiation of the dummy plate precursor, the photosensitive planographic printing plate precursor, the interleaving paper, and the stack board by a color sensor, and can be supplied into and conveyed in a CTP device without problems.
There also exists a need for a method for producing a planographic printing plate and a dummy plate for planographic printing in a common CTP plate-making system which method, even when planographic printing plate precursors and dummy plate precursor for planographic printings coexist in the system, enables easy distinguishing of the kind of a plate precursor selected from these precursors and determination of the processing suitable for the function of the plate precursor, also enables distinguishing of the base substrate and recognition of the fact that there is no planographic printing plate precursor or dummy plate precursor for planographic printing left, and can prevent deterioration in plate-making efficiency and elimination of tedious labor.